Boehm Titration - Carbon surface group characterization

[fangs]

I'm trying to analyze the surface oxygen functional groups on activated carbon using the Boehm titration method but I've been having problems due to differences reported in literature. I was wondering if anyone has performed this test before and could provide some guidance.

The method uses NaOH, Na2CO3, NaHCO3 and HCl and its main principle is that the number of acidic sites is determined under the assumptions that NaOH neutralizes carboxylic, lactonic and phenolic groups on the carbon surface; that Na2CO3 neutralizes carboxylic and lactonic groups; and that NaHCO3 neutralizes only carboxylic groups. The number of basic sites is calculated from the amount of HCl required in the titration.

I have done the titrations and calculated the amount of surface groups in mmol/g of carbon.

From the NaOH titration I found that there are 1.423 mmol/g of carboxylic, lactonic and phenolic groups.

From Na2CO3 titration there are 1.225 mmol/g of carboxylic and lactonic groups.

From NaHCO3 titration there are 1.575 mmol/g of carboxylic groups only.

So from the results there are 1.575mmol/g of Carboxylic groups, and by subtracting NaHCO3 titration and Na2CO3 titration this shows that there are 0.350 mmol/g of Lactonic groups. And finally the number of phenolic groups is calculated by subtracting the NaOH titration from the Na2CO3 titration to get 0.198mmol/g of phenolic groups.

Wouldn't it make sense that the NaOH titration would result in the highest amount in mmol/g since it neutralizes all 3 groups together? But from I've got, the highest amount was from the NaHCO3 titration 1.575mmol/g (only carboxylic groups neutralized)!!

[Markus Laeubli, Metrohm]

Dear fangs

I am not familiar with Boehm titration. But anyway as you write yourself hydroxide titrates all acid groups, hydrogen carbonate only the most acidic ones.
Therefore titrating one type of sample you need to get results in the way of R(OH) > R(CO3) > R(HCO3). Otherwise something went wrong. It does not make sense that you get the highest result with hydrogen carbonate. I also do not know any side reaction which could do this.

Just try again.

[Markus Laeubli, Metrohm]

I found a german thesis explaining that the Boehm titration is a partial titration only. Groups located in small pore might not react. I therefore think that also titration time is a critical point. Means that a short NaOH titration might show a lower result as a longer NaHCO3 titration.

[fangs]

Thank you very much for your interest and time to reply Dr. Markus.

I found the solution to my problem after several titrations which resulted in low NaOH. My sodium carbonate and bicarbonate were not in fact 0.05M as I had intended. After standardizing them it turned out to be much a lower molar concentration which caused the errors. It was rather a silly thing which I did not realize.

The thesis paper you found looks very interesting and I think there are somethings which I can use after translating to English. Thanks again.